The invention relates to a method and a device for dosing and packaging polysilicon chunks, and to a dosing unit and a packaging unit for a device for dosing and packaging polysilicon chunks.
Fragmented polysilicon is, for example, deposited from trichlorosilane by means of the Siemens method and then ideally comminuted contamination-free. A method for automatic breaking and a corresponding device are described in EP 1 645 333 A1.
For applications in the semiconductor and solar industries, minimally contaminated fragmented polysilicon is desired. For this reason, the material should also be packaged with low contamination before it is transported to the customer.
Conventionally, fragmented polysilicon for the electronics industry is packaged in 5 kg bags with a weight tolerance of +/−max. 50 g. For the solar industry, fragmented polysilicon in bags with a weigh-in of 10 kg and a weight tolerance of +/−max. 100 g is usual.
Tube bagging machines, which are suitable in principle for the packaging of fragmented silicon, are commercially available. A corresponding packaging machine is described, for example in DE 36 40 520 A1.
Fragmented polysilicon is a sharp-edged, non-flowable bulk material with a weight of up to 2500 g for the individual Si chunks. During packaging, it is therefore necessary to take care that the material does not pierce, or in the worst case even entirely destroy, the conventional plastic bags when they are being filled. In order to prevent this, commercially available packaging machines need to be suitably modified for the purpose of packaging polysilicon.
With commercially available packaging machines, it is generally not possible to comply with the purity requirements which are demanded of fragmented polysilicon, since the conventionally used composite films can lead to increased contamination of the fragmented polysilicon owing to the chemical additives.
EP 1 334 907 B1 discloses a device for the low-cost automatic transportation, weighing, portioning, filling and packaging of high-purity fragmented polysilicon, comprising a feed channel for the fragmented polysilicon, a weighing device for the fragmented polysilicon, which is connected to a funnel, deflection plates made of silicon, a filling device which forms a plastic bag from a high-purity plastic film and comprises a deionizer which prevents static charging and therefore particle contamination of the plastic film, a welding device for the plastic bag filled with fragmented polysilicon, a flowbox which is arranged above the feed channel, weighing device, filling device and welding device and prevents particle contamination of the fragmented polysilicon, and a conveyor belt with a magnetically inductive detector for the welded plastic bag filled with fragmented polysilicon, all the components which come in contact with the fragmented polysilicon being sheathed with silicon or clad with a highly wear-resistant plastic.
Means for portioning the fragmented polysilicon are, for example, a time-controlled feed channel or filling level determination of a storage container or a weighing device for the fragmented polysilicon. A corresponding weighing device is known, for example, from U.S. Pat. No. 4,813,205.
The device according to EP 1 334 907 B1 is intended to allow low-contamination packaging without human contact. The low-contamination packaging is intended, in particular, to be achieved by sheathing the internal components with silicon or with a highly wear-resistant plastic.
However, it has been found that specifically the portioning of fragmented polysilicon by a procedure according to EP 1 334 907 B1 is problematic. An exact 10 kg weigh-in of fragmented polysilicon with a tolerance of +/−100 g is not possible by means of this device. This applies in particular to chunks with sizes of 50-130 mm.
Furthermore, the overall arrangement has been found to be mechanically not very stable owing to the sheathing of all parts, which come in contact with silicon, with silicon or plastic. The relatively high wear of the silicon and plastic coatings makes the packaging machine extremely maintenance-intensive.
DE 10 2007 027 110 A1 discloses a device for packaging fragmented polycrystalline silicon or polysilicon granulate, consisting of a rotary machine, a filling machine and a sealing machine or a non-circularly arranged device comprising a filling station and a sealing station, in which a PE bag is suspended from a gripper system and moved from station to station in a timed sequence, characterized in that the filling station comprises a freely suspended energy absorber consisting of a low-contamination nonmetallic material, which is introduced into the PE bag before the PE bag is filled with polycrystalline silicon and is removed from the PE bag after the PE bag has been filled with polycrystalline silicon, and the filled PE bag is carried forward by means of the gripper system into the sealing station and sealed there.
DE 10 2007 027 110 A1 also describes a method for packaging polycrystalline silicon, in which a freely suspended ready-formed bag is filled with polycrystalline silicon by means of a filling device, the filled bag subsequently being sealed, characterized in that the bag consists of high-purity plastic with a wall thickness of from 10 to 1000 μm. Preferably, the sealed plastic bag filled with polycrystalline silicon is introduced into a further plastic bag made of PE with a wall thickness of from 10 to 1000 μm and this second plastic bag is sealed.
According to DE 10 2007 027 110 A1, the polysilicon is first portioned then weighed before the packaging. The portioning and weighing in of the fragmented polysilicon are carried out by means of a manual or automatic method known from the prior art. With respect to automatic portioning, the device known from EP 1 334 907 B1 is mentioned, although it has the disadvantages described above.
In order to achieve the required high weigh-in accuracy of less than +/−1% for fragmented polysilicon in the semiconductor industry, labor-intensive manual packaging of the cleaned polysilicon chunks in a class 100 cleanroom is necessary. In this case cleaned polysilicon chunks, which no longer have any metallic impurities on the surface, are taken using high-purity gloves, for example high-purity textile, PU or PE gloves, from a process vessel, in which the cleaning takes place, and introduced into a double PE bag. When handling with the gloves, however, the content of plastic and metal particles in the fragmented polysilicon increases owing to the glove abrasion and the general handling by the workers. Nevertheless, the manual method still satisfies the purity requirements in relation to the metal surface values for the electronics industry.
Furthermore, automatic weight correction is provided by re-weighing the filled and welded PE bags and removing the relevant bags if the weight is too high or too low, the weight being manually corrected for bags with the wrong weigh-in by optionally re-cleaning the polysilicon and filling a new bag with it, then welding the bag.
As an alternative, differential weighing of the process vessel is carried out before and after emptying; if there is a weight error of +/−50 g, the method is automatically stopped and the operators carry out manual correction. The PE bag is subsequently filled.
The welding of the PE bag is carried out according to DE 10 2007 027 110 A1 with a hot-seal welder, in which the metallic welding wire is clad with a nonmetallic material, for example polytetrafluoroethylene (PTFE).
It has been found that weld seams which have folds are often formed in this way. This is frequently the case in particular for the second bag and for chunks with a size of from 50 to 130 mm. For this reason, secure handling and secure transport to the customer cannot always be ensured.
The prior art thus often provides manual weight correction or even manual packaging of polysilicon, in order to be able to comply with the required weigh-in tolerances. Automatic dosing devices have proven mechanically unstable. Fold-free welding of the second bag is not achieved with the methods known from the prior art.
The object of the invention is based on these problems.